| Summary: | 博士 === 國立中央大學 === 地球物理研究所 === 91 === A complex source process inversion technique including three-dimensional fault geometry, three-dimensional Green''s functions and parallel non-negative least square inversion are used to explore the high resolution spatio-temporal slip distribution of the 1999 Chi-Chi, Taiwan, Earthquake. The 3D fault geometry was inferred from the aftershocks distributions and high-resolution reflection experiment studies. It shows dip angle of the fault becomes shallower from south to north and near flat at the deeper portion of the fault. The 3D Green’s functions were calculated from a 3D finite-difference code using 3D velocity structure derived from tomographic studies for grid size of 0.5km and time interval 0.05sec. The 3D Green’s functions show significant azimuthally variation of the waveform suggest the necessity of the 3D Green’s function calculations in Taiwan. On account of the 3D fault geometry and 3D Green’s functions, we use parallel linear inversion technique to invert the spatial/temporal slip distribution of the 1999 Chi-Chi earthquake using the dense strong motion waveforms considering huge amount of multiple time windows. Finally, the 3D source parameters derived from the inversion result was used to forward modeling the full-wave 3D wave propagation of the 921 earthquake. Our study here emphasizes the necessary of 3D fault geometry and 3D Green''s function that correctly account for both the complicated source process and wave propagation effect of the large damaged earthquake. The major achievements of this research are stated below :
A.High resolution 3D source process inversion
Final result is drive from 48 time-windows inversion. The rupture process shows very complex spatio-temporal slip behavior. Major slip area is located at the shallow part of the fault (about 10 km depth). The largest slip, 15.8m, is at about 30 km north of the epicenter which is near the bending of the fault and extend to about 15km depth. The rupture time history at southern and northern parts is entirely different. In southern part, the slip behavior is composed by repeatedly and small slip rate; while the northern part is dominated by single, duration about 10 sec, large slip rate event. Comparing with previous results, the resolution of the static slip distributions is more refined in this study. The same with the dynamic rupture process, there is some phenomena that can only be revealed detail in this study. These high resolution source process analyses can help to promote further study in earthquake physics and strong ground motion prediction.
B.Forward strong ground motion simulation of the 921 Chi-Chi earthquake
Joining the high resolution source process model and parallel 3D wave propagation technique, we have successfully reproduced the strong ground shaking during the Chi-Chi earthquake. Comparisons between the simulation results and observed waveforms from a dense strong motion waveforms clearly demonstrate that the variation of the velocity structure and the complex fault slip process greatly affect the damage during the Chi-Chi earthquake. The source directivity effect produced large amplitudes along the direction of the rupture propagation. Low velocity under shallow part of Coastal Plain generated significant amplified ground motion. Under Central Range, the seismic wave was relatively weak owing to the energy radiation mechanism of the Chelungpu fault. The ground motions were then amplified further by the high velocity gradient under Coastal Range. Finally, the strong ground motion characters at different regions were mainly dominated by intense source effects. The results of this research point out that a close link with strong motion observations, source studies, detailed knowledge of velocity structure, and numerical simulation technology is necessary to make the prediction of strong ground motion feasible in the further.
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